ABSTRACT The Gyeonggi Massif, Korea, consists of basement gneisses and supracrustal rocks migmatized to varying degrees. We conducted a petrologic-geochronologic study of the Mount Cheonggye gneisses, located in the western part of the Gyeonggi Massif, and we discuss the crustal evolution of the massif based on our results combined with a compilation of available data from the literature. Mineral assemblages and reaction textures in cordierite-garnet-biotite gneisses suggest a composite pressure-temperature path defined by two clockwise trajectories, M 1 and M 2 . Pseudosection modeling constrains M 1 peak metamorphic conditions as ~10.5 kbar and 840– 860 °C, followed by M 2 recrystallization at 4.5–5.5 kbar and 720–770 °C. Textural relationships of garnet to cordierite and kyanite to plagioclase transitions, as well as the pseudosection analysis, corroborate the clockwise pressure-temperature-time paths in the Gyeonggi Massif. We dated the polyphase metamorphism using sensitive high-resolution ion microprobe (SHRIMP) U-Pb data for zircon and monazite grains from eight samples. Overgrowth rims of zircon in a cordierite-garnet-biotite gneiss and a K-feldspar megacrystic orthogneiss yielded weighted mean 207 Pb/ 206 Pb ages of 1854 ± 9 Ma ( n = 11) and 1852 ± 12 Ma ( n = 19), respectively. This Paleoproterozoic age was reproduced by monazite grains from three cordierite-bearing gneisses dated at ca. 1861–1851 Ma. In contrast, monazite grains from a cordierite-bearing mylonitic gneiss and two biotite gneisses yielded consistent 206 Pb/ 238 U ages ranging from 235 ± 2 Ma ( n = 12) to 231 ± 2 Ma ( n = 15), suggesting a strong Triassic thermal overprint. Finally, we dated a postkinematic granitic dike at ca. 226 Ma, suggesting Late Triassic termination of the orogenesis. Our compilation of SHRIMP U-Pb ages from zircon, monazite, allanite, and titanite available from the literature confirms that the Gyeonggi Massif underwent two distinct thermal events in association with Paleoproterozoic (1.88–1.85 Ga) and Triassic (245–230 Ma) collisional orogenies. In contrast, Mesoproterozoic to Paleozoic thermal episodes are present in the Gyeonggi marginal belt, newly named in this study, where Neoproterozoic (ca. 950–750 Ma) and Paleozoic (ca. 450–430 Ma) ages are prominent in magmatic and detrital zircons. Our tectonic model, exemplified by the Qinling-Gyeonggi microcontinent, suggests that prolonged accretionary tectonics produced arc-related lithologies overlying the Gyeonggi Massif basement rocks. The juxtaposition of these terranes onto the Gyeonggi Massif produced tectonic mixtures with affinities to either the North or South China cratons. On the basis of similarities in zircon age distributions, we further suggest that the Qinling-Gyeonggi microcontinent is built upon basement rocks with North China craton affinity, at least in the Korean Peninsula and extending toward the Japanese Islands.

The blueschist-greenschist facies transition for a model basaltic system Na 2 O-CaO-MgO-Al2O 3 -SiO2-H 2 O is defined by a univariant reaction: 6 clinozoisite + 25 glaucophane + 7 quartz + 14 H 2 O = 6 tremolite + 9 chlorite + 50 albite; for the Fe 2 O 3 -saturated basaltic system, by a discontinuous one: 4 epidote + 5 Mg-riebeckite + chlorite + 7 quartz = 7 hematite (magnetite) + 4 tremolite + 10 albite + 7 H 2 O. These two reactions were experimentally investigated to determine the nature of the blueschist-greenschist transition. The results have located the first reaction at 350 ± 10°C, 7.8 ± 0.2 Kb and 450 ± 10°C, 8.2 ± 0.4 Kb. Reconnaissance experiments for the second reaction indicate that the minimum pressure for the occurrence of epidote + Mg-riebeckite + chlorite + quartz is about 4 Kb at 300°C for f O2 defined by the hematite-magnetite buffer. The presently determined P-T location for the blueschist-greenschist transition in the Fe-free basaltic system is about 3 Kb lower than the minimum pressure limit of glaucophane of Carman and Gilbert (1983), but is compatible with the revised stability field of jadeite + quartz determined by Holland (1980). Introduction of Fe 3+ into the model basaltic system significantly lowers the minimum pressure limit for occurrence of the buffered assemblage sodic amphibole + epidote + actinolite + chlorite + albite + quartz, and the participating phases gradually increase their Fe 3+ /Al ratio with decreasing pressure. Isopleths of sodic amphibole composition in the buffered assemblage in terms of X G1 are delineated and the effect of Fe 2+ and temperature on the isopleths are discussed. The Al 2 O 3 content of sodic amphibole coexisting with epidote + actinolite + chlorite + albite + quartz decreases systematically with decreasing pressure and hence can be used as a geobarometer. Pressure estimates for metabasites at Ward Creek of the Franciscan terrane, the Mikabu greenstones of the Sanbagawa belt, the Otago schists of Lake Wakitipu, New Zealand, and the blueschists at Ouegoa, New Caledonia, based on the proposed glaucophane geobarometry, are in agreement with those derived from sodic pyroxene geobarometry.